As is well known, semiconductor electronic devices comprise an electronic circuit formed in a small plate or "die" of a semiconductor material with a surface area dimension of a few square millimeters. Typically, the electronic circuit is integrated monolithically on a major surface of the die. The die is protected thermally and mechanically by encapsulating it within a package. Reference will be made in particular to plastic packages.
Such devices require suitable means of support and electrical interconnection for electrically connecting them to external circuits. For this purpose, a conductive lead frame, consisting of a thin wrought metal plate, comprises a plurality of narrow strips which represent electric connectors or leads each having one end connected electrically to the integrated circuit. Opposite, peripheral ends of the leads extend outwards from the package body for allowing electric interconnection of the device. On completion of the package forming process, the peripheral ends of the leads are bent and form respective pins for mounting the package, typically to a printed electronic board.
In the instance of integrated power circuits, that is, devices which typically produce a relatively large amount of heat for example due either to their high component density and/or number, or because intended for operation with large currents, reference is commonly made to so-called power packages. As used herein, the term "power package" indicates a device that can dissipate heat by some means of its own.
In a typical power package, a heat sink element or slug, supports the die and serves a heat dissipating function. The die is coupled thermally to the heat sink element to transfer outside the heat generated by the die during its operation.
The heat sink element typically is a metallic element or in any case a good heat conductor whose mass is definitely larger than that of the die or of the electronic device. It usually is in the form of a copper plate of non-negligible thickness.
In a typical realization, the semiconductor material die is mounted directly on a major surface of the heat sink element so that a surface of the die on which a circuit is formed remains free. The lead frame is attached to the heat sink element too.
Power packages are typically formed such that the heat sink element is embedded in the plastic package body, but has a major surface exposed, that is, uncovered by plastic material. The exposed major surface typically is a surface that is not contacting the die. This configuration promotes the transfer of heat from the device to the environment outside the package. To improve the dissipation of heat, this exposed major surface of the heat sink element may be in contact with an external heat sink of even larger size and mass, thereby providing enhanced heat dissipation. The external heat sink is typically formed by the structure itself to which the plastic case is soldered. Reference will be made throughout the following description and the drawings to the case in which the exposed major surface of the heat sink element is a bottom surface, that is, located on the same side as the bending of the pins. Alternatively, the top major surface of the heat sink element could be the exposed major surface, to which a suitable external heat sink may be attached.
Typically in a power package, the heat sink element has only one major surface exposed, the other major surface and all the side surfaces being insulated, that is covered with the plastic material of the case. Throughout the following description, side surfaces are those surfaces of the heat sink element which connect perimetrically the two major surfaces substantially parallel to each other.
Shown schematically in FIG. 1 is a package having an exposed heat sink element as it appears immediately after forming the plastic case. The package is shown in a side view taken parallel to the direction of the leads, and partly in section in the left-hand portion of the figure. The cross-section is the obliquely hatched area. The package is generally denoted by 1 in that figure.
FIG. 1 and the following figures illustrate by way of example a package of the DIP (Dual-In line Package) type, that is one having its leads projecting out from two opposite sides of the package body.
For clarity and simplicity of illustration, the semiconductor material die has been omitted from the sectional view.
A plastic case, shown at 2, has a heat sink element 3 embedded therein such that only the bottom major surface 4 thereof, at the same level as the case bottom surface, will be exposed. A leadframe 5 is connected to the heat sink element 3 at the opposite side from the surface 4 by connecting means such as rivets or weldings, not shown in the sectional view. The leadframe 5 is partly enclosed in the case 2, so that ends of its lead, denoted by 6, extend outside of the case 2, to the left and right in FIG. 1. Shown outside the case 2, getting out of the plane determined by the case side surface in view, is a support structure 7 of the leadframe interconnecting a number of leadframes, as explained hereinafter.
For a better understanding of aspects of the present invention the usual steps of a process for forming a conventional package with exposed heat sink will now be described briefly.
Each heat sink element is obtained by shearing from a relatively thick sheet of metal, e.g., copper. From a thinner sheet, also typically of copper, a bearing structure is formed in the form of a strip and a series of leadframes, each including an assembly of narrow metal strips which are interconnected by transverse connecting lines and are intended to become the leads. To the bearing structure at each leadframe is connected a corresponding number of heat sink elements.
A corresponding number of dies are mounted integrally to this formed structure, such that each die locates at one leadframe/heat sink element structure. As mentioned above and described, for example, in European Patent Application 545007 of this Applicant, each die is connected, according to a standard technique, directly to the heat sink element without the leadframe interposed therebetween. The die is positioned in the central region of the heat sink element, isolated from the leads. Reference will be made in the following description to this case wherein the leadframe has no dedicated area for accommodating the die. In this instance, the die is connected to the heat sink element either by soldering with an alloy such as tin/lead or by gluing with for example an epoxy glue. Alternatively, another solution provides for the die being accommodated on a central portion of the corresponding leadframe, which is also connected to the heat sink element. Thin wires, usually of gold or aluminum, are bonded with one end to dedicated metallized pads provided on the die surface where the integrated circuit is formed, and with the other end to the inward ends of the leads.
Subsequently, the strip with the dies mounted thereon is placed into a mold having cavities corresponding to the single devices for forming their corresponding plastic cases. Injected into each cavity is an electrically insulative material, in the molten state at a high temperature, which is to form the plastic body of the package. This material is typically a synthetic resin, e.g., an epoxy resin.
Molding in the proper sense comprises the injection of the resin into the cavities. However, the molding process comprises several phases whereat the temperature is varied gradually to avoid cracking the semiconductor material that forms the die, or otherwise making the overall device unreliable. The term molding is used herein to denote all the operations carried out inside the mold cavity: melting the plastic material, letting it expand into the cavity, and allowing it to solidify.
After an initial cooling step, and subsequent resin thermal curing steps to achieve thorough polymerization, the plastic cases will be completely formed, and the series of packages ready for removal from the mold.
FIG. 2 shows schematically a mold at a stage immediately preceding the molding step to form the package shown in the previous figure. The figure shows in particular a single mold cavity.
A mold for the injection of resin is generally shown at 8. It comprises, in the common embodiment shown in the figure, an upper half-shell or top mold designated 8a, and a lower half-shell or bottom mold 8b. Both half molds have a corresponding hollow, and when the mold is closed, are arranged with their hollows facing each other to provide a single mold cavity whereinto the resin can be injected.
The molten resin is injected through a gate type of inlet, designated 9 in FIG. 2, formed in the mold, specifically in the bottom mold 8b, which has one end open into the mold cavity. The gate inlet 9 has a substantially horizontal main axis. Its location is in any case such that the cavity can be filled even in its region at the extreme right in the figure, farthest from the resin entrance.
The structure that includes the heat sink element 3 and the leadframe 5 has been introduced for molding into the mold cavity, according to the configuration shown in FIG. 2. Also shown in FIG. 2 is a die of a semiconductor material 10 fixed to the center of the top surface of the heat sink element 3 opposite from the major surface 4 to be left exposed.
As can be seen, the leadframe 5 is located at the same level as the interface between the two half molds, and is positioned such that the terminating portions of the leads 6 are left outside the mold cavity. Thus, for forming such a package, the mold 8 typically has one closure level only, when viewed in vertical cross-section, consisting of the surfaces at the interface of the two half molds and corresponding to the level of the leadframe 5. The presence of such a closure level is effective to make possible the introduction of the leadframe into the cavity in a easy way while allowing its outermost portions to remain outside.
The heat sink element is located on the bottom of the mold cavity formed in the bottom mold 8b such that the bottom surface 4 will not be covered by the resin. On the other hand the mold cavity has larger side dimensions than the heat sink element 5, and will therefore fully enclose it so as to keep the heat sink element 5 isolated laterally all around its perimeter.
Both the bottom and top portions of the mold cavity have substantially the shape of a parallelepiped. Accordingly, the cavity shape for molding that type of package is particularly simple.
However, for certain applications, the power package has a different structure from that shown. For several necessities, the heat sink element, additionally to having a major surface exposed, will be partially extended laterally out of the plastic case, from at least one of its sides. For example, the heat sink element has two peripheral portions projecting from two opposed sides of the plastic case.
Thus, such packages also have exposed part of the other major surface and of the side surfaces of the heat sink element at such peripheral portions. This description will make reference to this type of package.
FIG. 3 is a perspective view of a known embodiment of a power package with the heat sink partially extended out of the plastic case. Reference will be made herein, by way of example, to a package known as Plastic Small Outline (PSO), of the type typically employed for automotive applications.
The package is overall denoted by the reference numeral 11 and shown at the end of its manufacture, after bending the pins, ready to be soldered to an external electric circuit.
As shown in the figure, the plastic case 2 partly encapsulates a heat sink element, generally denoted by 12 here, which, besides having its bottom surface exposed, extends sidewards out of the plastic case 2 in the direction denoted by y. On the contrary in the orthogonal x direction, the dimensions of the plastic case 2 exceed those of the corresponding sides of the heat sink element 12.
In particular, the heat sink element 12 has in this example two peripheral portions, designated 13 in FIG. 3, which symmetrically extend outwards. In these portions 13, the top major surface 14, the side surface 15 lying in the x direction, and part of the side surfaces 16 parallel to the direction determined by the axis y are also exposed.
The profile of the plastic case 2 and of the heat sink element 12 are determined by the particular conformation of the specific example.
The leads, bent over to form the pins 17, are extended outside the plastic case 2 of the package 11 from opposite sides in the x direction, orthogonally to the direction along which the heat sink element 12 extends beyond the case 2.
For forming the plastic case of a package of this type, a mold must be used which has a more complex molding cavity than that shown in the previous FIG. 2.
The term main cavity will be used in the following description to indicate that portion of the mold cavity which has the shape of the plastic case envelope without taking into account the presence of the heat sink element therein. Thus, it will commonly have a substantially parallelpipedic shape. The main mold cavity, such as the mold cavity used in the example shown in FIG. 2, typically results from the combination of an upper hollow formed in the top mold and a lower recess provided in the corresponding half-shell.
In this case, however, the dimension of the main mold cavity along the y direction must be shorter than the length along y of the heat sink element 12. Thus, it will be necessary for the mold considered overall to comprise a specific seat for the heat sink element, and in particular to provide a housing for the heat sink element which extends in the y direction outside the main mold cavity, and which is furthermore connected to the main mold cavity such that the plastic case can be formed around the heat sink element in the x direction. This housing is a blind hole, i.e., without side outlets, formed in the bottom portion of the mold.
The molding technique provides for the mold to have for this purpose, additionally to a conventional first closure level corresponding to the leadframe, as in the simpler case previously discussed, a second closure level at the top major surface of the heat sink element. The aforementioned housing is provided at least below this level.
Such a configuration may typically cause some drawbacks during the molding step. Specifically, problems may be encountered at the interface between the housing portions which are intended to receive the portion of the heat sink element outside the plastic case, and the main mold cavity. In fact, at the second closure level the mold sealing during the molding is specially critical. Particularly between the surfaces of the heat sink element, which must be left exposed, and said housing formed in the mold, hermeticity is very difficult to achieve. Thus, during the molding step, the molten resin may partly leak out of the mold cavity in the y direction towards the most peripheral portions of the heat sink element. Over the related surfaces of the heat sink element, the leaking resin may result in the formation of flash, that is thin layers of plastic material which solidify on the external surfaces of the heat sink element.
To illustrate this problem, FIG. 4 shows a schematical top view of the lower portion of a mold during the molding of the package shown in the previous figure. Both the leadframe and the semiconductor material die have been omitted, and only the heat sink element 12 of the semiconductor structure is shown, for clarity reasons.
The lower portion of the mold main cavity, denoted by 18, extends in the x direction sideways of the heat sink element 12 in connection with its central portion. The heat sink element 12 is disposed to have its peripheral portions 13 at the ends of a housing 19, which is formed in the mold lower portion and extends substantially in the y direction beyond the mold main cavity 18. The upper closure of the housing 19 is determined by the upper structure of the mold, not shown in FIG. 4.
FIG. 4a is an enlarged detail view of the portion enclosed within a broken line circle in FIG. 4.
As can be seen, it is impossible to arrange for perfect adhesion of the portions of the side surfaces 16 of the heat sink element that are to stay out of the case to the walls of the housing 19. To indeed ensure proper positioning of the heat sink element 12 in the housing 19 with no cracking risk, the latter is formed with its width along x in the portions 13 slightly smaller than that of the cavity which forms the housing. Thus, a passage channel on the order of a few microns is left at each of the portions 13. An arrow indicates in FIG. 4a a flow direction of a resin leak from the mold main cavity toward the housing 19. Upon solidification of the resin, thin resin layers or flashes will form on the surfaces of the portion 13.
For the device to operate properly, an additional step must be provided after molding to remove the resin flashes. However, those resin flashes are very difficult to remove because of their positions.
On the other hand known are molds whose inner shape presents into the cavity elements which push a metal slug against the bottom of the housing where it is placed, so as to improve the adhesion of the heat sink element to the mold. However, not even this configuration is appropriate to prevent resin formations on the exposed side surfaces of the heat sink element, although it ensures good adhesion of the housing bottom to the bottom surface of the heat sink element.
A known solution to this problem provides for the use of blocking bars formed in the structure of the heat sink element at the peripheral portions thereof. To this aim, the heat sink element/leadframe structure is formed integrally from a thick metal sheet, by shaping the heat sink element and then reducing the thickness at the sides and forming by shearing the leadframe.
A structure of this type is shown in FIG. 5. The heavy outlined portion is the heat sink element 12, lighter lines being used to draw the leadframe 5. As shown in the figure, the leadframe 5 and heat sink element 12 form a mono-component and are joined to each other by blocking bars 20. These bars 20 of copper consist of side extensions of the peripheral portions 13 of the heat sink element 12 in the x direction. When the structure of FIG. 5 is placed into the mold, the bars 20 substantially close the side channel for the resin flow indicated in FIG. 4a.
On completion of the molding step the bars 20 must be removed to separate the heat sink element 12 along the broken lines in the figure. However, this additional operation renders the process critical because cracks may occur at the side extensions due to inaccuracies of the parting apparatus, which requires appropriate calibration.
A further drawback of the known solution of FIG. 5 is related to the formation of the mono-component heat sink/leadframe structure. The leadframe thickness cannot be less than a minimum value, if it is not to be distorted during its formation due to the weight of the heat sink element whose mass is far greater.
In addition, with this known solution, the mold must be given quite a complex configuration in order to accommodate the mono-component structure.
While reliability and repeatability of a package thus formed can be improved, still the formation of resin flashes cannot be prevented altogether.
A further known solution is disclosed in U.S. Pat. No. 5,445,995 to the assignee of the present invention. The mold is modified there to allow the resin to flow freely further outwards the peripheral portions of the heat sink element along a y direction into suitable alveoli.
FIG. 6 shows schematically a vertical cross-section along the y direction through a mold made in accordance with that patent. Thrust elements 21 compress the heat sink element against the bottom of its housing. As shown, alveoli 22 outside the heat sink element are filled with resin through suitable channels, not shown. The width of each alveolus 22 in the x direction is the same as that of the portions of the heat sink element which are to be left outside the plastic case.
The plastic tips thus formed within the alveoli 22 must then be removed, after taking the case out of the mold. This additional operation is critical especially with configurations like that shown in FIG. 3 by way of example, wherein the peripheral portions of the heat sink element have the same width as the heat sink element, and substantially of the same order as the plastic case. In this instance, the plastic case may be cracked during that step.
Thus, it is impossible to form a package which will be free from resin residues on the exposed surfaces of the heat sink element, without this being accompanied by drawbacks.
The underlying problem of this invention is to provide a method whereby can be formed, by only the molding process without the need of additional steps, a plastic package with an heat sink exposed and having portions extending peripherally from the plastic case kept free from resin leakouts.
Furthermore the mold used and the conformation of the heat sink element/leadframe structure should be particularly simple.